1. Field of the Disclosure
This disclosure relates generally to control systems for power distribution equipment. More specifically, this disclosure relates to control system actions to prevent a motor known as a “close motor” in a network protector unit from burning out. Network protectors and the close motor for network protectors can be used in both spot networks and grid networks.
In order to provide context it is useful to start with a simplified network diagram.
A simplified drawing is provided in FIG. 1 to introduce certain relevant components. As the focus of this application is on the protection of the network protector close motor, many relevant components that would be present in an actual power grid distribution system have been omitted.
A portion of an electrical distribution network is shown as network 100. Network 100 has feeder bus 104, feeder bus 108, and feeder bus 112. A representative voltage for operation of these feeder buses may be 13 Kv but other systems may operate at 27 Kv, 34 Kv or some other voltage. The power on these three buses is provided to a set of local distribution networks 116 to serve loads represented by 120, 124, and 128. The voltage on these local distribution networks is apt to be 120 volts, but it could be 277 volts, 341 volts or some other voltage. In some cases these loads represent a building or even a portion of a very large building. Depending on the amount of load, the local distribution network may be coupled to one, two, or three feeder buses (104, 108, and 112). Even when the load can consistently be serviced by just one feeder bus, a desire for reliability leads to providing a redundant path for providing service in case of equipment failure, scheduled maintenance, load balancing, or other needs. Actual networks may have more than three possible feeder buses connected to a particular local distribution network but simplified FIG. 1 is sufficient for introducing the concepts relevant to the present invention.
The local distribution networks 116 are coupled to the feeder buses 104, 108, and 112 through transformers 150 and related equipment. The transformers 150 convert the relatively higher voltage on the primary side 154 of the transformers 150 to the relatively low voltage on the secondary side 158 of the transformers 150.
The transformers 150 are connected to the feeder buses (104, 108, and 112). The feeder buses may be isolated from the network by breakers 162. Primary fuse links 174 may exist adjacent to the breakers 162. The transformers 150 have network protectors 166 on the secondary side 158 of the transformers 150 to isolate the transformers 150 from the local distribution networks 116 as needed to protect the transformers from current flowing from the distribution networks 116 to the primary side 154 of the transformers (known as back feed). The network protectors 166 are frequently contained in a submersible enclosure bolted to the throat of the transformer 150 which is placed in underground vaults. Network protectors 166 are an important component in a distribution network. The network protectors are subject to IEEE standard C57.12.44.
Additionally, some networks include sets of fuse links 170 between the network protectors 166 and the local distribution networks 116.
The feeder buses 104, 108, and 112 can be isolated by a set of substation breakers 178 from the substation buses 182 that are the next portion of the transmission network which is ultimately connected to a set of power sources that drive generators. The power sources are symbolically represented here by turbine 190.
FIG. 1 shows a small portion of the network which may have more feeder buses and many more local distribution networks 116 providing power to many more loads. These loads may be distributed around a portion of a city. The various transformers 150 may be in pits (vaults) near the various loads. Thus it is convenient to inject analog signals onto the power lines so that the analog signals can be picked off by pick-up coils from the feeder buses at the substation and fed to a receiver 186. These analog signals are often in the frequency range of 40 kHz to 70 kHz which is much higher than the frequency of the power being distributed over the network. (For example one common frequency for power grids is 60 Hertz although other frequencies are used throughout the world and can be used in connection with the present invention).
A preferred location for injecting the analog signal containing information about the operation of a transformer and related equipment is on the secondary side 158 of the transformer between the network protector 166 and the load. Transmitter 130 is shown in FIG. 1 to illustrate this location. Placement of transmitter 130 in this location allows for the injection of the analog signal onto the relatively low voltage, secondary side of the transformer 150. Traversing the transformer 150 from secondary side to primary side provides only a slight attenuation of the high frequency carrier signal.
2. FIG. 2
FIG. 2 provides additional detail on the network protector 166. Network protector 166 has a network protector relay that is shown here in logical components to facilitate discussion. Network protector relay logic 204 operates to open or close network protector breaker 208. Network protector relay logic 204 has access to voltage indications by voltage measuring equipment (not shown) connected to secondary side connections 212 for each of the three phases and to load side connections 216. Alternatively, the network protector relay logic may have access to one set of voltages and one set of differential voltages. Other sets of data to accomplish the same end would be apparent to one of skill in the art.
The network protector relay logic 204 may also have access to output from a set of three current transformers (not shown) or a single output from an aggregating current transformer. In either case, the network protector relay logic 204 can detect the flow of current towards the transformer 150 from the local distribution network and intervene to open the network protector breaker 208. The network protector relay logic 204 may also have access to information about the phase angles of the power for each phase on either side of the network protector breaker 208. The network protector relay logic 204 may automatically close the network protector breaker 208 when appropriate to connect the secondary side of the transformer 150 to the local distribution network 116.
The network protector relay logic 204 may respond to local operation of rules or to an external request to open or close the network protector breaker 208.
The network protector breaker 208 is opened through the energy stored in springs (not shown) that are loaded with energy as the network protector breaker 208 is driven closed by close motor 220. The system to allow the springs to trip the network protector breaker 208 to open may be in communication with a trip relay (not shown). The trip relay may be normally energized and a de-energized status of the trip relay causes a trip. Thus, a loss of power by the network protector relay logic 204 will cause the trip relay to become de-energized and fail-safe by opening the network protector breaker 208.
Close motor 220 is energized by the network protector relay logic 204 in order to close the network protector breaker 208. A close contact 240 under control of the network protector relay logic 204 selectively applies power from a power supply to the close motor 220.
The close motor 220 may not operate the network protector breaker 208 directly but may be connected to a spring (not shown here) that is loaded and used to rapidly close the network protector breaker 208. Typically, one close motor 220 is used to close all three phases. The close motor may be a small single phase commutator high torque motor.
Repeated use of the close motor 220 in quick succession as the network protector breaker 208 continues to reopen can cause the close motor 220 to burn out and become inoperable. The prior art network protector relay logic 204 monitored for pumping where “pumping” is the repeated successful closing and opening of the network protector breaker 208.